Our long-term goal is to understand the organization of the brainstem auditory pathways in terms of connections among the multiple neuronal types that form them. The objective of the proposed studies is an analysis of the origins and terminations of specific pathways that interconnect the midbrain and forebrain auditory nuclei of the gerbil (Meriones unguiculatus). Neuroanatomical tracing studies are proposed to achieve two specific aims: 1) To confirm and extend our recent finding that the central nucleus of the inferior colliculus gives rise to at least two distinct pathways to the ventral division of the medial geniculate nucleus, and 2) To determine whether the two parts of the ventral division of the medial geniculate defined by the sources of their collicular projections project differentially to the auditory cortex. The working hypothesis that underlies our specific aims is that information is carried from the brainstem into the forebrain along multiple, largely separate parallel pathways. The separate pathways presumably play different functional roles in auditory processing. We propose to use neuroanatomical methods based on the anterograde and/or retrograde axonal transport of tracer molecules specifically chosen for their suitability in each experiment. The potential impact of the work on public health is indirect but potentially far-reaching. Neuroanatomical data provide the essential structural framework for relating results of studies in the many disciplines applied to the auditory system, from molecular approaches to behavioral testing. The neuronal activity that leads to normal functional expression is dependent on the precise and specific connections among hundreds of specialized neuronal types. Detailed understanding of these connections will contribute to the development of rational, testable hypotheses about auditory function in both health and disease.